The invention relates to a method for producing liquid pig iron or liquid steel intermediate products from fine-particled material containing iron oxide. The fine-particled material containing iron oxide is prereduced in at least one prereduction stage with the aid of a reduction gas and is subsequently reduced in a final reduction stage to a sponge iron. The sponge iron is melted in a melt-down gasification zone, with carbon carriers and oxygen-containing gas being supplied. A CO- and H2-containing reduction gas is generated. That gas is introduced into the final reduction stage, is converted there, is drawn off, is subsequently introduced into at least one prereduction stage, is converted there and is drawn off. The invention relates, further, to an apparatus for carrying out this method.
A method of this type is already known, for example, from EP 969 107 A1. In this, in order to reduce fine ore to sponge iron, usually a plurality of fluidized bed reactors are used which are arranged in a cascade. The fine ore particles have a very pronounced tendency to agglomeration during reduction in the fluidized bed reactors. This effect occurs to an increased extent with a rising degree of fineness of the ore particles, with rising reduction gas temperatures and with a rising degree of metallization. For this reason, it has been possible hitherto to implement industrially, and with acceptable outlay in terms of cleaning and maintenance and acceptable availability of the plant, only those fluidized bed processes which were being operated with a relatively low degree of metallization of the sponge iron of about 70% or with coarser fine ore (0-10 mm) and reduction gas temperatures of below 800° C. for the last fluidized bed reactor and of between 700° C. and 760° C. for the prereduction reactors. However, a relatively low reduction gas temperature entails the disadvantage of a correspondingly lower introduction of heat. In addition, heat losses at the individual fluidized bed reactors and the connecting pipelines between the fluidized bed reactors occur which are higher, comparatively, than in the reduction of lump ore in a reduction shaft. The reduction gas temperature required for an optimal metallization process in the individual fluidized bed reactors connected in series can therefore be maintained to only a restricted extent without the supply of additional energy.
This energy deficit can be compensated by means of various additional measures. The energy introduction which is additionally necessary can be covered by means of a higher specific reduction gas quantity per ton of burden or by means of an additional partial combustion of CO and H2 by oxygen being injected. In addition to a higher introduction of sensible heat, what is also achieved by increasing the specific reduction gas quantity is that less CO and H2 has to be burnt in the connecting lines between the fluidized bed reactors, in order to raise the reduction gas temperature to an optimal value for the following fluidized bed reactor. Further, what is achieved by an increased specific reduction gas quantity is that, owing to the partial combustion of CO and H2, the reduction potential of the reduction gas does not undershoot a predetermined limit value for the fluidized bed reactor following in each case, so that a relatively high degree of metallization, even of larger iron ore particles requiring a longer dwell time in the reducing atmosphere than the smaller ore particles, is achieved.
In a melt reduction method in one or more fluidized bed reactors connected in series, such as is known, for example, from EP 969 107 A1, the reduction gas required for reducing the iron oxides and for calcining the aggregates is produced in a melt-down gasifier, using coal as a gasification agent and oxygen or oxygen-enriched air as oxidizing agent. The heat occurring during the gasification process is used in the melt-down gasifier for melting sponge iron and the necessary aggregates into pig iron and slag which are tapped at specific time intervals. When a melt-down gasifier is coupled to a reduction shaft furnace, as takes place in the COREX® method, if coals with a fraction of volatile constituents of above 27% are used, the reduction gas quantity produced in the melt-down gasifier is sufficient for a steady-state operation of the reduction shaft. When a melt-down gasifier is coupled to fluidized bed reactors, optimized operation can be achieved only by means of additional measures already described above, such as a higher specific reduction gas quantity and partial combustion of CO and H2.
So that the fluidized bed reactors can be operated with higher specific reduction gas quantities, the possibility is likewise already known of either drawing off purified blast furnace gas from the fluidized bed reactors of a CO2 removal plant and delivering it to the reduction process again or operating the melt-down gasifier with highly volatile coals. The use of gasification agents with a high fraction of volatile constituents and with a low calorific value requires very high specific consumptions of gasification and oxidizing agents and entails very high specific slag rates and is therefore uneconomical. For technical and economic reasons, therefore, the production of increased reduction gas quantities via the gas processing plant is preferred. However, due to the relatively low output of blast furnace gas (off gas) in a CO2 removal plant, in the case of coals with a low fraction of volatile constituents, the additional gas quantity generated according to this method is not sufficient for producing the sponge iron with relatively uniform metallization, particularly not in the case of low specific fuel consumptions, such as are desired per se.